Current wireless communication systems are typically configured to include a core network, such as a Remote Network Controller (RNC), which is coupled to one or more base stations such as Node Bs, which in turn, are coupled with a plurality of WTRUs.
FIG. 1 shows the mapping of logical channels to the RACH/PRACH in a Third Generation (3G) system in the Time Division Duplex (TDD) mode. It should be understood by those of skill in the art that the mapping scheme in FIG. 1 is not the mapping scheme in TDD, as there are other mapping schemes for mapping channels to DCH/DPCH in TDD. Control information, such as that information transmitted over the common control channel (CCCH) and the dedicated control channel (DCCH) is mapped to the RACH. These logical channels are employed for radio resource control (RRC) connection requests, cell information updates, UTRAN registration area (URA) updates and radio bearer establishment and reconfiguration. In addition, non-real time (NRT) traffic from the dedicated traffic channel (DTCH) and the shared common control channel (SHCCH) are also mapped to the RACH. The RACH is then mapped to the physical channel and becomes the physical RACH (PRACH) channel.
Many current communication systems have uplink common channels (i.e., channels which handle communications transmitted from a WTRU to the Node B) which are accessible by all WTRUs. These channels are used to establish and maintain a wireless connection between the WTRU and the Node B for transmitting both control information and data. The random access channel (RACH) of a 3G system in the TDD mode is such a channel. The RACH is defined as an uplink contention-based common transport channel. When two or more WTRUs attempt to transmit their respective information over the RACH channel at the same time, a contention may occur. To alleviate the contention problem, each WTRU waits a different random amount of time before retransmitting its message to the Node B.
A WTRU having information to transmit over the RACH performs a random back-off process. When a WTRU has a block of data to transmit over the RACH, it performs the random back-off process to access the RACH. More specifically, before the start of a frame, the WTRU randomly generates a number, uniformly distributed between 0 and 1. It then compares the number to a threshold called the dynamic persistence level (DPL), which is also a value between 0 and 1 (for example, 0.5). If the generated random number is less than the DPL, then the WTRU transmits the block of data over the RACH. If the generated random number is greater than the DPL, the WTRU waits until the next frame, at which point it generates a new random number and repeats the process. The WTRU will wait to access the RACH until it has a successful comparison between the random number and the DPL.
In some current systems, the RNC acts as a central controller and controls the rate at which WTRUs access the RACH (and therefore control the duration of the back-off process and the likelihood of collisions) by varying the DPL. The central controller generally has no prior knowledge of which WTRUs, if any, have transmitted over the RACH/PRACH. To make it more difficult for WTRUs to access the RACH, the RNC reduces the dynamic persistence level, for example, from 0.5 to 0.25, making it less probable that the random number generated by the WTRU at a given frame will be smaller than the DPL. By making it more difficult to access the RACH, the probability of there being a collision between multiple WTRUs decreases.
On the other hand, the DPL may be increased (for example, from 0.5 to 0.75) in order to make it easier for WTRUs to access the RACH. By increasing the DPL, it is more probable that the random number generated by the WTRU at a given frame will be smaller than the DPL. This results in a shorter back-off process, but a higher probability of collision between WTRUs.
The RACH is mapped onto the PRACH for transmission. The detection of transmitted PRACH codes is performed at the Node B by midamble detection and code lookup. When PRACH codes are detected, a cyclic redundancy check (CRC) is performed at the Node B to detect errors within the received transmission. A transmission error can result from either a code collision, in which multiple WTRUs transmit using the same PRACH code, or from insufficient transmission power. The PRACH is typically defined as one code within a code in a timeslot. Typically, multiple PRACHs are defined within the same timeslot, or the entire timeslot is reserved for PRACH codes. The Node B monitors the energy level individually for each PRACH (i.e., for each code/timeslot combination that is a PRACH). In this manner, the Node B detects individually, for each PRACH, whether there was an attempt and whether or not it was successful. Therefore, PRACH codes for which successful and failed transmissions occurred are known at the Node B at each frame.
However, there is currently no simple and fast acknowledgment mechanism from the Node B to the WTRU for confirmation of successful or unsuccessful PRACH/RACH attempts. The WTRU must wait for higher layers to process the signal and determine whether or not a transmitted burst was successfully received. When a PRACH transmission fails, the radio link control (RLC) entity, the RRC or some other higher layer entity typically observes the absence of a response for a period of time before the data is retransmitted. In some implementations, a timer specifies the duration before retransmission. The delay incurred for a successful RACH transmission is significantly influenced by the latency incurred in the event of a transmission error. As a result, excessive delays have been observed for the successful transmission of data due to retransmissions.
It would be desirable to have a fast feedback mechanism for notifying WTRUs of the success or failure of a transmission using PRACH/RACH codes. Such a feedback mechanism should be extremely fast, have backwards compatibility and have a low complexity.